Expansion valve control for heat transfer system

ABSTRACT

A compressor is connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid in a system for recovering waste heat to provide heated water for human use. A suction superheat temperature is determined from a measured compressor suction temperature and a suction saturation temperature. The electrically controlled valve is adjusted to maintain the suction superheat temperature at a suction superheat set point. The electrically controlled valve may be incrementally closed when a compressor suction pressure exceeds a maximum suction pressure. A discharge superheat temperature can be determined from a measured compressor discharge temperature and a discharge saturation temperature. The electrically controlled valve may be incrementally closed when the discharge superheat temperature falls below a minimum discharge superheat temperature.

FIELD

The present invention relates to heat transfer systems.

BACKGROUND

It is known to employ energy exchange technologies in order to, for example, recover excess heat energy from an air-conditioning system to provide energy to heat water. Many examples of such heat-exchange technologies came about in the early 1980s which reflect the end of the energy crises of the 1970s. It is interesting to note that these heat-exchange technologies have not been generally adopted.

Existing solutions do not provide precise and robust control adequate for heat recovery systems, given that waste-heat recovery typically has large temperature gradients of the kind unforgiving to poor control.

SUMMARY

According to one aspect of the present invention, a heat transfer system includes a compressor for circulating a working fluid, the compressor having an inlet and an outlet, a condenser connected to the outlet of the compressor, an electrically controlled valve positioned to receive working fluid from the outlet of the condenser, an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor, a suction pressure sensor located between the outlet of the electrically controlled valve and the inlet of the compressor, a suction temperature sensor located at the inlet of the compressor, and a controller connected to the suction pressure sensor, the suction temperature sensor, and the electrically controlled valve. The controller is configured to adjust the electrically controlled valve to maintain output of the suction pressure sensor and the suction temperature sensor above a saturation point of the working fluid.

The controller can be configured to adjust the electrically controlled valve to maintain output of the suction temperature sensor at a suction superheat set point that is based on a suction saturation temperature that the controller determines from output of the suction pressure sensor.

The controller can be configured to adjust the electrically controlled valve to maintain output of the suction pressure sensor to below a maximum suction pressure.

The system can further include a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve, and a discharge temperature sensor located at the outlet of the compressor.

The controller can be configured to adjust the electrically controlled valve to maintain output of the discharge pressure sensor and the discharge temperature sensor at above saturation of the working fluid.

The controller can be configured to adjust the electrically controlled valve to maintain output of the discharge temperature sensor above a minimum discharge superheat temperature based on a discharge saturation temperature that the controller determines from output of the discharge pressure sensor.

The system can further include a subcooler connected between the condenser and the electrically controlled valve.

The evaporator can be configured to receive flow of waste-heat bearing fluid.

The condenser can be configured to receive flow of water to be heated.

According to another aspect of the present invention, a method of controlling a heat transfer system includes determining a suction saturation temperature of a compressor, the compressor connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid. The method further includes measuring a suction temperature for the compressor, determining a suction superheat temperature from the measured suction temperature and the suction saturation temperature, and adjusting the electrically controlled valve to maintain the suction superheat temperature at a suction superheat set point.

The method can further include determining a suction pressure of the compressor, and incrementally closing the electrically controlled valve when the suction pressure exceeds a maximum suction pressure.

The method can further include determining a discharge saturation temperature of the compressor, measuring a discharge temperature for the compressor, determining a discharge superheat temperature from the measured discharge temperature and the discharge saturation temperature, incrementally closing the electrically controlled valve when the discharge superheat temperature is below a minimum discharge superheat temperature.

The method can further include feeding waste-heat bearing fluid to the evaporator.

The method can further include feeding water to the condenser and outputting heated water from the condenser.

According to another aspect of the present invention, a heat transfer system for heating water using waste heat includes a compressor for circulating a working fluid, the compressor having an inlet and an outlet and a condenser connected to the outlet of the compressor. The condenser is configured to receive flow of water to be heated. The system further includes an electrically controlled valve positioned to receive working fluid from the outlet of the condenser and an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor. The evaporator is configured to receive flow of waste-heat bearing fluid. The system further includes a suction pressure sensor located between the outlet of the electrically controlled valve and the inlet of the compressor, a suction temperature sensor located at the inlet of the compressor, a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve, and a discharge temperature sensor located at the outlet of the compressor. The system further includes a controller connected to the suction pressure sensor, the suction temperature sensor, the discharge pressure sensor, the discharge temperature sensor, and the electrically controlled valve. The controller is configured to adjust the electrically controlled valve to maintain output of the suction temperature sensor at a suction superheat set point above a suction saturation temperature determined from output of the suction pressure sensor, except when one or more of output of the suction pressure sensor exceeds a maximum suction pressure and output of the discharge temperature sensor falls below a minimum discharge superheat temperature determined from output of the discharge pressure sensor, in which case the controller incrementally closes the electrically controlled valve.

A plurality of the systems can operate at different pressures, and water to be heated flows from the condenser of a lower pressure system to the condenser of a higher pressure system.

In addition, water to be heated can flow in parallel through subcoolers of the lower pressure system and the higher pressure system before flowing into the condenser of the lower pressure system.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate, by way of example only, embodiments of the present invention.

FIG. 1 is a diagram of a heat transfer system according to an embodiment of the present invention.

FIG. 2 is a pressure-enthalpy chart for the working fluid and the heat transfer system.

FIG. 3 is a block diagram of control logic of the controller.

FIG. 4 is a block diagram of decision logic of the controller.

FIG. 5 is a diagram of a heat transfer system according to another embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a heat transfer system 10 according to an embodiment of the present invention. The heat transfer system may be known as a heat pump, refrigeration loop, or similar. The heat transfer system provides precise and robust control, particularly when used in waste-heat recovery and water heating for human use.

The heat transfer system 10 includes a compressor 12, a condenser 14, an electrically controlled expansion valve 16, and an evaporator 18 connected together in a loop by fluid conducting piping. A working fluid is provided to the system 10. The working fluid can include refrigerants, such as R-134a, R-12, R-124a, R-401a, R-404a, R-409A, R-414A, or similar.

The compressor 12 is a screw-type compressor that circulates the working fluid in the system 10. The compressor 12 has an inlet for receiving working fluid in a low-pressure vapor state, and an outlet for discharging compressed working fluid as a high-pressure vapor. In other embodiments, the compressor is another kind of compressor.

The condenser 14 has an inlet connected to the outlet of the compressor 12, and has an outlet that feeds the electrically controlled valve 16. The condenser 14 can be configured to receive water or other fluid to heat. In this embodiment, cold water 22 flows into the condenser 14 and leaves the condenser 14 as hot water 24. For example, cold water 22 arrives at between 10 and 55 degrees Celsius and is heated to hot water 24 at between 40 and 70 degrees Celsius. Other temperatures are also possible. These example temperatures are conducive to heating water for residential or hotel use for cleaning, washing, cooking, or bathing. Cold water 22 may be potable and may originate from a municipal supply, from a re-circulating hot water tank, from a boiler feed line, or similar.

The electrically controlled valve 16 is positioned to receive at its inlet condensed working fluid from the outlet of the condenser 14. The electrically controlled valve 16 may be known as an ETX valve. The electrically controlled valve 16 can include a stepper motor and gear assembly configured to position a pin in the port through which working fluid flows, so as to incrementally open or close the port to increase or decrease flow of working fluid. The electrically controlled valve 16 creates a controllable pressure drop in the working fluid, thereby expanding the working fluid into a mixed vapor-liquid state at its outlet. Control of the valve 16 controls the pressure drop and thus the exiting quality, temperature, and pressure of the working fluid.

The evaporator 18 is connected between the outlet of the electrically controlled valve 16 and the inlet of the compressor 12. The evaporator 18 can be configured to receive a heat-bearing medium, such as water, an alternative liquid, or a gas. In this embodiment, waste-heat bearing fluid 26, such as that available from air-conditioning systems, enters the evaporator 18 and discharges its heat to the working fluid, before leaving the evaporator 18 as cooled fluid 28. The temperature of the arriving waste-heat bearing fluid 26 may be between about 10 and 50 degrees Celsius. Other temperatures are also possible.

The system 10 may further include a subcooler 32 connected between the condenser 14 and the electrically controlled valve 16. Flow of working fluid through the subcooler 32 may discharge heat to very cold water 34, having a temperature below the temperature of the cold water input to the condenser 14. Warmed water exiting the subcooler 32 may be fed into the condenser 14 as cold water 22.

The system 10 further includes a suction pressure sensor 42 located between the outlet of the electrically controlled valve 16 and the inlet of the compressor 12. In this embodiment, the suction pressure sensor 42 is located near the inlet of the compressor 12. The specific location of the suction pressure sensor 42 can be varied, provided that the pressure drop expected between the location of the suction temperature sensor 44 and the compressor 12 is taken into account.

The system 10 further includes a suction temperature sensor 44 located at the inlet of the compressor 12.

The system 10 further includes a controller 50 connected to the suction pressure sensor 42, the suction temperature sensor 44, and the electrically controlled valve 16. The controller 50 can include a processor, memory, input interface, and output interface. The controller 50 is configured to adjust the electrically controlled valve 16 to maintain output of the suction pressure sensor 42 and the suction temperature sensor 44 at levels above a saturation point of the working fluid.

FIG. 2 shows a pressure-enthalpy chart for the working fluid. No specific working fluid is depicted. However, the chart applies to at least those working fluids mentioned herein. Isothermals are shown in dashed line.

The controller 50 is configured to adjust (e.g., incrementally open or close) the electrically controlled valve 16 to maintain output of the suction temperature sensor 44 at a suction superheat temperature 62. To achieve this, a suction superheat set point 64 is set above the saturation point of the working fluid. Maintaining the suction superheat temperature 62 to be at the suction superheat set point 64 can prevent the evaporator 18 from overheating the working fluid, which may detrimentally affect output of the compressor 12 and cost compressor power, and may also prevent under-heating the working fluid, which can advantageously prevent liquid-state working fluid from entering the compressor 12.

The controller 50 determines compressor inlet saturation temperature from output of the suction pressure sensor 42 and subtracts the determined saturation temperature from the output of the suction temperature sensor 44 to determine the actual suction superheat temperature 62. The controller 50 employs a suction superheat control loop to maintain the suction superheat temperature 62 at the suction superheat set point 64 by controlling the electrically controlled valve 16. Example values for the suction superheat set point 64 include 3-5 degrees Kelvin, and similar values above saturation suitable for a safety margin above saturation. The suction superheat set point 64 is a differential temperature relative to the saturation temperature and so can be expressed in relative units such as Celsius or Fahrenheit or absolute units such as Kelvin or Rankine.

In operation, when the heat input from the waste-heat bearing water 26 decreases, the system 10 may tend to output lower temperature working fluid at the evaporator 18, which may bring the working fluid exiting the evaporator 18 towards a saturated state. The risk of saturation at the compressor inlet is reduced or prevented by the controller 50 maintaining the suction superheat temperature 62 at the suction superheat set point 64.

The controller 50 may also be configured to incrementally close the electrically controlled valve 16 to maintain the output of the suction pressure sensor 42 to below a maximum suction pressure 65. This can advantageously maintain the suction pressure below the suction pressure limit of the compressor, particularly when the temperature of waste-heat bearing water 26 is relatively high. The maximum suction pressure 65 can be expressed in units of pressure or as a maximum saturation temperature, with output of the suction pressure sensor 42 being converted to saturation temperature to allow comparison.

Referring back to FIG. 1, the system 10 can further include a discharge pressure sensor 46 located between the outlet of the compressor 12 and the inlet of the electrically controlled valve 16. In this embodiment, the discharge pressure sensor 46 is located near the outlet of the compressor 12. The specific location of the discharge pressure sensor 46 can be varied, provided that the pressure drop expected between the location of the discharge pressure sensor 46 and the outlet of the compressor is taken into account. The system 10 can further include a discharge temperature sensor 48 located at the outlet of the compressor 12.

The controller 50 can be further configured to incrementally close the electrically controlled valve 16 to maintain output of the discharge pressure sensor 46 and the discharge temperature sensor 48 at levels above saturation of the working fluid.

Referring again to FIG. 2, the controller 50 is configured to incrementally close the electrically controlled valve 16 to maintain output of the discharge temperature sensor 48 at a discharge superheat temperature 66 that is above a minimum discharge superheat temperature 68. This can advantageously maintain the discharge superheat, particularly on start-up when the system 10 is cold or when the suction pressure is high and the discharge pressure is low. Operating the compressor 12 too close to saturation at discharge can result in liquid-state working fluid entering the lubricating oil system of the compressor 12. This problem is particularly evident in semi-hermetic screw-type compressors, which permit working fluid to enter the oil separator and may allow working fluid to cool significantly at discharge. Thus, the compressor discharge is controlled by maintaining the discharge superheat temperature 66 at least a minimum discharge superheat temperature 68 amount above the saturation point of the working fluid.

The controller 50 determines compressor discharge saturation temperature from output of the discharge pressure sensor 46 and subtracts the determined saturation temperature from the output of the discharge temperature sensor 48 to determine the actual discharge superheat temperature 66. The controller 50 employs a discharge superheat control loop to maintain the discharge superheat temperature 66 at above the minimum discharge superheat temperature 68 by incrementally closing the electrically controlled valve 16. Example values for minimum discharge superheat temperature 68 include 20-25 degrees Kelvin, and similar values above saturation sufficient to prevent working fluid from cooling excessively inside the compressor 12 where it may contaminate lubricating oil and reduce the service life of the compressor 12. The minimum discharge superheat temperature 68 is a differential temperature relative to the saturation temperature and so can be expressed in relative units such as Celsius or Fahrenheit or absolute units such as Kelvin or Rankine.

Discharge pressure of the compressor 12 can be allowed to float based on control using the suction superheat set point 64, maximum suction pressure 65, and the minimum discharge superheat temperature 68. The controller 50 is thus configured to adjust the electrically controlled valve 16 to maintain evaporator 18 pressure as high as practical, while not exceeding the suction pressure limit of the compressor 12 and also while preventing saturated working fluid from condensing in the oil separator of compressor 12.

It can be seen from FIG. 2 that the system 10, when applied to waste heat recovery for heating residential or hotel water, operates at a relatively high end of the thermodynamic cycle for the working fluid. This allows efficient use of commonly available and safe working fluids to recover waste heat.

FIG. 3 illustrates control logic resident in the controller 50. The control logic can implement the methods and other techniques described herein. As such, the control logic may take the form of a specialized computer program, a group of parameters inputted into a preprogrammed control routine, or the like.

Output from the suction pressure sensor 42 is converted to a saturation temperature 82 at the inlet of the compressor 12. The can be performed with reference to a lookup table 84 that stores relationships between saturation pressures and saturation temperatures for the working fluid. The measured suction temperature from the suction temperature sensor 44 is reduced by the suction saturation temperature 82 to arrive at the actual suction superheat temperature 62.

The actual suction superheat temperature 62 and the suction superheat set point 64 are provided as inputs to a suction superheat control loop 88 whose output is a suction superheat valve command 90 for adjusting the electrically controlled valve 16. The suction superheat valve command 90 is a change in valve position that brings the actual suction superheat temperature 62 towards the suction superheat set point 64. The suction superheat set point 64 can be inputted by an operator of the system 10. The actual suction superheat temperature 62 and the suction superheat set point 64 can be expressed as true temperatures on a standard scale (e.g., 25 degrees Celsius) or as temperatures relative to saturation temperature (e.g., 5 degrees Celsius or Kelvin, or by convention “5K”). It is expected that such an incremental change in the valve 16 position is an incremental opening or closing of the valve 16.

Similarly, output from the discharge pressure sensor 46 is converted to a saturation temperature 92 at the outlet of the compressor 12 with reference to the lookup table 84. The measured discharge temperature from the discharge temperature sensor 48 is reduced by the discharge saturation temperature 92 to arrive at the actual discharge superheat temperature 66. The actual discharge superheat temperature 66 and the minimum discharge superheat temperature 68 are provided as inputs to a discharge superheat control loop 96 whose output is a discharge superheat valve command 98 for adjusting the electrically controlled valve 16. The discharge superheat valve command 98 is an incremental change in the valve 16 position that keeps the actual discharge superheat temperature 66 above the minimum discharge superheat temperature 68. It is expected that such an incremental change in the valve 16 position is an incremental closing of the valve 16. The minimum discharge superheat temperature 68 can be inputted by an operator of the system 10. The superheat temperatures 66, 68 can be expressed in a standard scale (e.g., 80 degrees Celsius) or as relative temperatures (e.g., 20K).

An operator-adjustable maximum suction pressure 65 and the output of the suction pressure sensor 42 are taken as inputs to a suction pressure control loop 102, which outputs a suction pressure valve command 104 representing an incremental change in the valve 16 position that keeps the measured suction pressure below the maximum suction pressure 65. It is expected that such an incremental change in the valve 16 position is an incremental closing of the valve 16.

The control loops 88, 96, 102 may each be PI, PID, or P feedback control loop that provides error output representative of an incremental valve opening or closing value. In this embodiment, the control loops 88, 96, 102 are PI feedback control loops.

Decision logic 106 determines which of the valve commands 90, 98, 104 to send to the electrically controlled valve 16 as the actual valve command 108. In this embodiment, the decision logic 106 selects the valve command 90, 98, 104 that requests the largest increment of closing. If no valve command 90, 98, 104 requests an incremental closing of the valve 16, then the control logic selects the suction superheat valve command 90. This results in the ignoring of any incremental open requests from the discharge superheat valve command 98 and the suction pressure valve command 104. In other words, the controller 50 incrementally adjusts the valve 16 based on the suction superheat set point 64, unless the discharge superheat temperature 68 falls below its minimum 68 or the suction pressure 42 exceeds its maximum 65, in which case the controller 50 incrementally closes the valve 16 by the maximum closing increment requested. That is, the suction superheat control loop 88 controls the valve 16, unless incremental valve closing is requested by either or both of the suction pressure control loop 102 and the discharge superheat control loop 96, in which case control of the valve is passed to the control loop 102, 96 or 88 requesting largest incremental amount of valve closing.

FIG. 4 illustrates an example embodiment of the decision logic 106, assuming that incremental close commands are represented by negative values and incremental open commands are represented by positive values. The lowest value out of the suction superheat valve command 90, the suction pressure valve command 104, and the discharge superheat valve command 98 is selected at 120. If the lowest value is not negative, as determined at 122, then the value of the suction superheat valve command 90 is taken, at 124, as the output valve command 108. If the lowest value is negative, then, at 126, the lowest value is taken as the output valve command 108 to control the valve 16 to incrementally close.

The control process illustrated in FIGS. 3 and 4 repeats in real time or near real time, as the system 10 operates.

The controller 50 may further provide for an alarm shutdown if any of the sensors 42-48 detects an abnormal condition on one of the control loops.

FIG. 5 illustrates another embodiment of a heat transfer system 130 according to the present invention. The system 130 is similar to the system 10 and only differences will be discussed in detail. For description of other features and aspects of the system 130, description of the system 10 can be referenced, with like numerals identifying like components.

The heat transfer system 130 uses two of the heat transfer systems 10, one a low-pressure system 134 to provide initial heating to water and another a high-pressure system 136 to provide further heating to the water.

The evaporators 18 may each receive waste-heat bearing fluid 26 and output cooled fluid 28. The subcoolers 32 may be fed in parallel with very cold water 32, which is warmed at 22 and then fed through the low-pressure system's condenser 14 before being fed through the high-pressure system's condenser 14, so that the water is progressively heated. Heated water 24 exits the first condenser 14 and further heated water 138 exits the second condenser 14.

A controller 132 controls operation of the low-pressure system 134 and the high-pressure system 136. The systems 134, 136 may use different working fluids and may be controlled at different pressures and temperatures. However, the principles of control are the same as discussed above.

The controller 132 operates using the teachings discussed herein for the control 50. That is, the controller 132 references the compressor suction temperature and pressure for each system 134, 136 to adjust the respective electrically controlled valve 16 to maintain the working fluid at the inlet of each of the compressors 50 at a respective suction superheat set point. At the same time, the controller 132 may reference compressor suction pressure for each system to incrementally close the respective electrically controlled valves 16 to maintain each suction pressure to below a respective maximum suction pressure. Further, the controller 132 may control the discharge temperature and pressure for each system 134, 136 to adjust the respective electrically controlled valve 16 to keep the working fluid at the outlet of the compressor 50 above a respective minimum discharge superheat temperature.

The suction superheat set points, the maximum suction pressures, and the minimum discharge superheat temperatures may be different or the same for each of the low-pressure system 134 and the high-pressure system 136. For example, the suction superheat set points and the minimum discharge superheat temperatures may be the same in both the low-pressure system 134 and the high-pressure system 136, while different maximum suction pressures may be used for the systems 134, 136. Other examples are also contemplated.

In view of the above, it should be understood that the control techniques and systems described herein are precise, robust, and efficient, and particularly well suited for control of heat transfer systems used for waste heat recovery to heat water for human use in cooking, cleaning, bathing and other activities.

While the foregoing provides certain non-limiting example embodiments, it should be understood that combinations, subsets, and variations of the foregoing are contemplated. The monopoly sought is defined by the claims. 

What is claimed is:
 1. A heat transfer system comprising: a compressor for circulating a working fluid, the compressor having an inlet and an outlet; a condenser connected to the outlet of the compressor; an electrically controlled valve positioned to receive working fluid from the outlet of the condenser; an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor; a suction pressure sensor located between the outlet of the electrically controlled valve and the inlet of the compressor; a suction temperature sensor located at the inlet of the compressor; and a controller connected to the suction pressure sensor, the suction temperature sensor, and the electrically controlled valve, the controller configured to adjust the electrically controlled valve to maintain output of the suction pressure sensor and the suction temperature sensor at above a saturation point of the working fluid.
 2. The system of claim 1, wherein the controller is configured to adjust the electrically controlled valve to maintain output of the suction temperature sensor at a suction superheat set point that is based on a suction saturation temperature that the controller determines from output of the suction pressure sensor.
 3. The system of claim 1, wherein the controller is configured to adjust the electrically controlled valve to maintain output of the suction pressure sensor to below a maximum suction pressure.
 4. The system of claim 1, further comprising: a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve; and a discharge temperature sensor located at the outlet of the compressor.
 5. The system of claim 4, wherein the controller is configured to adjust the electrically controlled valve to maintain output of the discharge pressure sensor and the discharge temperature sensor at above saturation of the working fluid.
 6. The system of claim 5, wherein the controller is configured to adjust the electrically controlled valve to maintain output of the discharge temperature sensor above a minimum discharge superheat temperature based on a discharge saturation temperature that the controller determines from output of the discharge pressure sensor.
 7. The system of claim 1, further comprising a subcooler connected between the condenser and the electrically controlled valve.
 8. The system of claim 1, wherein the evaporator is configured to receive flow of waste-heat bearing fluid.
 9. The system of claim 8, wherein the condenser is configured to receive flow of water to be heated.
 10. A method of controlling a heat transfer system, the method comprising: determining a suction saturation temperature of a compressor, the compressor connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid; measuring a suction temperature for the compressor; determining a suction superheat temperature from the measured suction temperature and the suction saturation temperature; and adjusting the electrically controlled valve to maintain the suction superheat temperature at a suction superheat set point.
 11. The method of claim 10, further comprising: determining a suction pressure of the compressor; and incrementally closing the electrically controlled valve when the suction pressure exceeds a maximum suction pressure.
 12. The method of claim 10, further comprising: determining a discharge saturation temperature of the compressor; measuring a discharge temperature for the compressor; determining a discharge superheat temperature from the measured discharge temperature and the discharge saturation temperature; and incrementally closing the electrically controlled valve when the discharge superheat temperature is below a minimum discharge superheat temperature.
 13. The method of claim 10, further comprising feeding waste-heat bearing fluid to the evaporator.
 14. The method of claim 13, further comprising feeding water to the condenser and outputting heated water from the condenser.
 15. A heat transfer system for heating water using waste heat, the system comprising: a compressor for circulating a working fluid, the compressor having an inlet and an outlet; a condenser connected to the outlet of the compressor, the condenser configured to receive flow of water to be heated; an electrically controlled valve positioned to receive working fluid from the outlet of the condenser; an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor, the evaporator configured to receive flow of waste-heat bearing fluid; a suction pressure sensor located between the outlet of the electrically controlled valve and the inlet of the compressor; a suction temperature sensor located at the inlet of the compressor; a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve; a discharge temperature sensor located at the outlet of the compressor; and a controller connected to the suction pressure sensor, the suction temperature sensor, the discharge pressure sensor, the discharge temperature sensor, and the electrically controlled valve, the controller configured to adjust the electrically controlled valve to maintain output of the suction temperature sensor at a suction superheat set point above a suction saturation temperature determined from output of the suction pressure sensor, except when one or more of output of the suction pressure sensor exceeds a maximum suction pressure and output of the discharge temperature sensor falls below a minimum discharge superheat temperature determined from output of the discharge pressure sensor, in which case the controller incrementally closes the electrically controlled valve.
 16. A heat transfer system comprising a plurality of the systems of claim 15 operating at different pressures, in which water to be heated flows from the condenser of a lower pressure system to the condenser of a higher pressure system.
 17. The heat transfer system of claim 16, wherein water to be heated flows in parallel through subcoolers of the lower pressure system and the higher pressure system before flowing into the condenser of the lower pressure system. 